The "used" medium from the cultivation of normal diploid fibroblasts contains a macro-molecule (30,000 to 50,000 daltons) which secifically inhibits the proliferation of fibroblasts in vitro. This endogenous mitotic inhibitor appears to be cell-specific, non-cytotoxic, and its activity is reversible; thus, it appears to be a fibroblast chalone. This fibroblast chalone is thermolabile, destroyed by trypsin, and has an a cid isoelectric point. This chalone will significantly inhibit the proliferation of human osteosarcoma cells in culture. We are attempting to purify and characterize both chemically and biologically this fibroblast chalone and hope to find evidence of its existence in fibroblast-rich tissues.